I couldn't install OCaml in Termux, so there is workaround which uses x86-64 binaries and QEMU:
pkg install qemu-user-x86-64
curl -LO https://github.com/bcpierce00/unison/releases/download/v2.53.7/unison-2.53.7-ubuntu-x86_64-static.tar.gz
mkdir unison-unpacked
tar xvzf unison-2.53.7-ubuntu-x86_64-static.tar.gz -C unison-unpacked
cp unison-unpacked/bin/unison unison
cp unison-unpacked/bin/unison-fsmonitor unison-fsmonitor-bin
cat << EOF > unison-fsmonitor